Macrophages play a central role in human immune responses and defense against infection. Macrophages originate from blood monocytes, which leave the circulation in response to several signals that are thought in include chemoattractants elaborated at foci of inflammation by tissue leukocytes stimulated by invading microorganisms or by tissue injury. Heretofore, no pure, human leukocyte derived monocyte attracting substance has been provided.